1. Field of the Invention
The present invention relates to an ink jet printing apparatus that prints on print media by ejecting ink onto it.
2. Description of the Related Art
The ink jet printing apparatus ejects ink droplets from a print head to form images on print media. This ink jet printing apparatus can easily be upgraded to increase a printing speed and enable high-density printing and color-image printing. It also has an advantage of low noise during the printing operation.
The ink jet print head has a plurality of ink ejection openings and liquid paths communicating to the individual ink ejection openings. Each of the liquid paths is provided with a printing element for ejecting ink present in the liquid path from the ink ejection opening. The printing element is formed of an energy conversion element that transforms an electric energy into an ink ejection energy. Among the popular printing elements currently in use are, for example, an electrothermal conversion element (heater) that transforms an electric energy into a thermal energy to eject ink and an electromechanical conversion element (piezoelectric element) that transforms an electric energy into a mechanical energy for ink ejection.
The print head of a type that utilizes heat produced by the electrothermal conversion elements in ejecting ink from the ejection openings applies a voltage to each heater to generate heat. This heat energy boils ink in the ink path to produce a bubble which in turn ejects ink from the ejection opening. In the following description in this specification, a portion including the ink ejection opening, the ink path communicating to the ink ejection opening and the printing element installed in each ink path is called a nozzle.
With the ink jet printing apparatus using such a print head, however, variations are likely to occur among the printing elements such as heaters and piezoelectric elements. Applying a fixed amount of energy to all printing elements without taking such variations into account may result in ink droplets ejected differing in volume or printing elements having different longevities. To deal with this problem, it is a conventional practice, performed before shipping a print head from a factory, to measure an optimal threshold of ejection energy and, based on the threshold, write an optimum value of ejection energy in a memory incorporated in the print head. This allows a user during a printing operation to apply an optimal drive energy to the printing elements for ink ejection.
However, there are variations in a voltage of power supplied to the user for the ink jet printing apparatus and also in a drive voltage for the print head. This means that the optimal value of the drive energy, that was written into the print head during its manufacture, may become deviated out of an appropriate range because of the drive voltage variations. A technology to eliminate variations on the ink jet printing apparatus side is disclosed in Japanese Patent Laid-Open Nos. 2001-239658 and 2000-225698.
The technology disclosed in the Japanese Patent Laid-Open Nos. 2001-239658 and 2000-225698 is as follows. First, measurement patches are formed for each level of heater drive energy by changing it. Next, a density of each of the formed patches is read by a sensor. Then, the drive energy supplied when a blurred patch was formed is set as a threshold energy. Based on the threshold energy, an optimal drive energy to be supplied to the heater is set.
The technology disclosed in the Japanese Patent Laid-Open Nos. 2001-239658 and 2000-225698, however, has the following problem.
That is, in the currently used ink jet print head that has a growing demand for increased density and number of nozzles, the number of nozzles that are driven simultaneously to form a test pattern tends to increase. This in turn may cause a large voltage drop in a current supply circuit to the heaters, resulting in fluctuations of the heater drive voltage. If that happens, the optimum value of the drive energy to be supplied to the heaters becomes difficult to determine precisely.
To deal with this problem, a method may be conceived which, to make a voltage drop unlikely, reduces the number of nozzles driven simultaneously to form a test pattern. Since the number of nozzles used is reduced, the number of dots forming the test pattern also decreases, lowering the density of the patch. The reduced density of the patch results in a slower rate at which the density changes until the patch becomes blurred. Therefore, if a check is made of the blurring condition of the patch by using an ordinary sensor with a low detection precision, a result of the decision made may have large errors. That is, there exists almost no difference in density between a correct pattern, that is printed with a drive energy close to the one used when the patch becomes blurred, and a blurred pattern. As a result, there is a possibility of erroneously determining a correct pattern as a blurred pattern. To avoid this problem a sensor with high accuracy may be used. A high-precision sensor, however, is expensive leading to a cost increase of the printing apparatus.